A number of hydrocarbons, especially lower-boiling light hydrocarbons, in formation fluids or natural gas are known to form hydrates in conjunction with the water present in the system under a variety of conditions—particularly at a combination of lower temperature and higher pressure. The hydrates usually exist in solid forms that are essentially insoluble in the fluid itself. As a result, any solids in a formation or natural gas fluid are at least a nuisance for production, handling and transport of the same. It is not uncommon for hydrate solids (or crystals) to cause plugging and/or blockage of pipelines or transfer lines or other conduits, valves and/or safety devices and/or other equipment, resulting in shutdown, loss of production and risk of explosion or unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, hydrocarbon hydrates have been of substantial interest as well as concern to many industries, particularly the petroleum and natural gas industries.
Hydrocarbon hydrates are clathrates, and are also referred to as inclusion compounds. Clathrates are cage structures formed between a host molecule and a guest molecule. A hydrocarbon hydrate generally is composed of crystals formed by water host molecules surrounding the hydrocarbon guest molecules. The smaller or lower-boiling hydrocarbon molecules, particularly C1 (methane) to C4 hydrocarbons and their mixtures, are more problematic because it is believed that their hydrate or clathrate crystals are easier to form. For instance, it is possible for ethane to form hydrates at as high as 4° C. at a pressure of about 1 MPa. If the pressure is about 3 MPa, ethane hydrates can form at as high a temperature as 14° C. Even certain non-hydrocarbons such as carbon dioxide, nitrogen and hydrogen sulphide are known to form hydrates under the proper conditions.
There are two broad techniques to overcome or control the hydrocarbon hydrate problems, namely thermodynamic and kinetic. For the thermodynamic approach, there are a number of reported or attempted methods, including water removal, increasing temperature, decreasing pressure, addition of “antifreeze” to the fluid and/or a combination of these. Thermodynamic techniques function by shifting the hydrate formation equilibrium to a point outside the hydrate-forming conditions exhibited in the fluid. Kinetic inhibitors operate within hydrate equilibrium conditions. The kinetic approach generally attempts (a) to prevent the smaller hydrocarbon hydrate crystals from agglomerating into larger ones; (b) to inhibit the hydrocarbon hydrates from being formed in the first place; (c) to slow down crystal formation or growth under a particular set of conditions; and/or a combination of these approaches.
Kinetic efforts to control hydrates have included use of different materials as inhibitors. For instance, the use of compounds normally referred to as “quats” has been described in, inter alia, EP-A-736130, EP-A-824631, U.S. Pat. No. 5,648,575 and WO-A 98/05745. The “quat” type compounds focus around quaternary onium, in particular quaternary ammonium, compounds containing two or three lower alkyl chains, preferably containing C4 and/or C5 alkyl groups and one or two longer alkyl chains, preferably containing at least eight carbon atoms, which are bound to the central nitrogen moiety, thus forming a cationic species which is matched by a suitable anion such as a halide or other inorganic anion. Preferred “quats” comprise two long chains, comprising between 8 and 50 carbon atoms, which may also contain ester groups and/or branched structures. Additives such as polymers with lactam rings have also been employed to control clathrate hydrates in fluid systems. These kinetic inhibitors are commonly labelled Low Dosage Hydrate Inhibitors (LDHI) in the art.
WO 01/77270 discloses the use of dendrimeric compounds as hydrate inhibitors. Dendrimeric compounds are in essence three-dimensional, highly branched oligomeric or polymeric molecules comprising a core, a number of branching generations and an external surface composed of end groups. A branching generation is composed of structural units that are bound radially to the core or to the structural units of a previous generation and which extend outwards. The structural units have at least two reactive monofunctional groups and/or at least one monofunctional group and one multifunctional group. The term multifunctional is understood as having a functionality of 2 or higher.
An object of the invention is to provide an improved method for inhibiting gas hydrate formation in mixtures of hydrate-forming guest molecules and water where hydrates would otherwise form to a greater extent in absence of the method.
Another object of the invention is to provide gas hydrate inhibitor compositions and/or hydrate inhibitor synergists that are readily produced.
Therefore, the present invention provides a method for inhibiting formation of hydrocarbon hydrates in a mixture comprising water and hydrate-forming guest molecules, the method comprising contacting the mixture with a composition which comprises at least one dendrimeric compound effective to inhibit formation and/or agglomeration of hydrates in the mixture having a number average molecular weight of at least 1,000 atomic mass units (amu); and
at least one small molecular weight species having a molecular weight of less than 1,000 amu, selected from the group consisting of polyalkyleneimine, polyallylamine, starch, sugars, and polymers or copolymers of vinyl alcohol or allyl alcohol, where the composition amount is effective in inhibiting formation of the hydrocarbon hydrates in the mixture. The method may involve contacting the mixture with the composition under conditions effective to form the hydrocarbon hydrates in the absence of the composition. The composition amount is effective in inhibiting formation of the hydrocarbon hydrates in the mixture.